The success of cellular telephone systems has led to increased demand. To provide the communication bandwidth needed to satisfy this increased demand, technologies have been developed that enable several conversations to occur simultaneously in a single frequency band. In time division multiple access (TDMA), each communication frequency is divided into “time slots”, each time slot being used for a separate conversation. A second technique relies on code division multiple access (CDMA) to increase the number of users that can share any given communication frequency. In such systems each user encodes his or her transmissions with a code that is orthogonal, or nearly orthogonal, to the codes used by the other users of that communication frequency. In effect, the communication band is divided into a number of separate channels that can transmit information simultaneously. The maximum number of channels that can share any given communication frequency depends on the levels of interference and noise in each channel.
A significant fraction of the interference and noise experienced by each channel originates from the transmissions that are geographically localized such as transmissions from other users of the communication frequency. In the ideal case, the channels are perfectly separated, and hence, transmissions in one channel should not “leak” into the other channels. In the case of CDMA, the degree of channel isolation depends on a property of the codes used to encode the transmissions. Codes that are “orthogonal” to one another provide channels that do not leak into one another. Unfortunately, even in those cases in which mathematically orthogonal codes are utilized, the implementation of those codes on commercially feasible equipment and the dispersive nature of the propagation environment lead to some interchannel leakage. In effect, the imperfect electronics and propagation environment converts even perfectly orthogonal codes to codes that are only nearly orthogonal
One method for reducing the interchannel leakage is to utilize “adaptive smart antenna processing” to discriminate against interference and noise sources that are at geographically distinct locations with respect to the signal source of interest. Adaptive smart antenna processing utilize arrays of antennae together with adaptive signal processing to reduce the apparent signal strength from interference sources that are geographically separated from the cellular user of interest while enhancing the signal from that user. Consider two CDMA cellular users that are transmitting on the same frequency with different codes. As noted above, each user's signal will “leak” into the other user's communication channel. If sufficient distance separates the users from each other, and the antennae complex utilizes adaptive smart antenna processing, then the cell can be configured to transmit and receive in a spatially selective manner that reduces the interchannel leakage. In such a system, the base station (BS) configures the antennae complex to aim its signals to the first user when sending signals to that user and to receive signals preferentially from the location of that user. The transmission and reception pattern is also chosen such that there is a null region at the location of the second user. That is, the transmission energy received at the location of the second user is much lower than that received at the location of the first user when the BS is transmitting to the first user. Similarly, the apparent signal strength of the second user is significantly reduced when the BS is configured to listen to the first user. Hence, even if a small fraction of the second user's signal leaks into the first user's channel, the leakage is substantially reduced in strength since the apparent strength of the second user's signal has been reduced.
One form of adaptive smart antenna processing is referred to as linear adaptive smart antenna processing. In a linear adaptive smart antenna processing system, the signal from each antenna in the array is first amplified and phase shifted. The resultant signals are then added together to form a signal that may be viewed as being generated by a “virtual” antennae that enhances signals from a specific location within the cell while attenuating signals from other locations. The amplification factors and phase shifts determine the acceptance pattern for the virtual antenna. Similarly, a signal can be preferentially beamed to a localized region in the cell by generating an amplified and phase shifted signal from that signal to be applied to each of the antennae.
Since each user could be at a different location, a set of parameters is associated with each user. These parameters determine the phase shifts and amplification factors used in communicating with that user. The parameter set defines a “downlinking” set of phase shifts and amplification factors for use in communications from the BS to the cellular user and an “uplinking” set of phase shifts and amplification factors for use in communications from the cellular user to the BS. In general, these parameters can be determined “experimentally” for each user by searching the space of possible phase shifts and amplifications for the set that provides the best signal quality for that user. Since users and interference sources move and the environment changes with time, the process must be repeated at regular intervals.
There is a considerable investment in digital cellular units that employ CDMA. Accordingly, it would be advantageous to add the benefits of adaptive smart antenna processing to such systems without requiring that the cellular users obtain new equipment or reprogram existing cellular telephones. Ideally, only those BSs requiring additional capacity would need to be altered by introducing antennae arrays and the associated signal processing hardware. Hence, any method for evolving existing CDMA systems to combined CDMA-adaptive smart antenna processing systems should not require any new features in the existing cellular handsets.
The uplinking parameters for receiving a signal from a cellular user can be determined without the involvement of the cellular user. For example, the uplinking parameters may be determined at the BS by searching for the set of phase shifts and amplification factors that provide the best reception for the user in question. Several methods are known for determining the phase shifts and amplification factors that optimize the signals received at the BS, and hence, these methods will not be discussed in detail here. The reader is referred to U.S. Pat. Nos. 5,515,378 and 5,642,353 entitled “SPATIAL DIVISION MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEMS”, to Roy, et al., based on methods that use the direction of arrival of the signal from the cellular user. U.S. Pat. Nos. 5,592,490 entitled “SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS”, to Barratt, et al., and 5,828,658 entitled “SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING”, to Ottersten, et al., describe methods based on spatial and spatio-temporal signatures, respectively. U.S. Pat. No. 5,989,470 entitled “METHOD AND APPARATUS FOR DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING”, to Barratt, et al., describes a decision directed reference signal based method. These patents are hereby incorporated by reference.
If communications from the BS to the cellular user are performed at the same frequency as those from that user to the BS, then the downlinking parameters can be readily computed from the uplinking parameters. This computation also requires calibration data related to the various electronic components in the signal amplification paths and the characteristics of the antennae. Unfortunately, CDMA systems that conform to the IS-95 standard use different frequencies for the donwlinking and uplinking communications with each user. As a result, the optimum downlinking parameters cannot in general be readily determined without some involvement by the cellular handset.
Broadly, it is the object of the present invention to provide an improved method for operating an adaptive smart antenna processing-CDMA cellular telephone system to determine the downlinking signal processing parameters.
It is another object of the present invention to provide a method for determining the downlinking signal processing parameters without requiring that any new features be added to existing CDMA cellular handsets.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.